Allow retrieving the relations of a redacted event. (#12130)

This is allowed per MSC2675, although the original implementation did
not allow for it and would return an empty chunk / not bundle aggregations.

The main thing to improve is that the various caches get cleared properly
when an event is redacted, and that edits must not leak if the original
event is redacted (as that would presumably leak something similar to
the original event content).

3 files changed, 42 insertions, 33 deletions

diff --git a/synapse/storage/databases/main/cache.py b/synapse/storage/databases/main/cache.py
index abd54c7dc7..d6a2df1afe 100644
--- a/synapse/storage/databases/main/cache.py
+++ b/synapse/storage/databases/main/cache.py
@@ -191,6 +191,10 @@ class CacheInvalidationWorkerStore(SQLBaseStore):
 
         if redacts:
             self._invalidate_get_event_cache(redacts)
+            # Caches which might leak edits must be invalidated for the event being
+            # redacted.
+            self.get_relations_for_event.invalidate((redacts,))
+            self.get_applicable_edit.invalidate((redacts,))
 
         if etype == EventTypes.Member:
             self._membership_stream_cache.entity_has_changed(state_key, stream_ordering)
diff --git a/synapse/storage/databases/main/events.py b/synapse/storage/databases/main/events.py
index 1dc83aa5e3..1a322882bf 100644
--- a/synapse/storage/databases/main/events.py
+++ b/synapse/storage/databases/main/events.py
@@ -1619,9 +1619,12 @@ class PersistEventsStore:
 
         txn.call_after(prefill)
 
-    def _store_redaction(self, txn, event):
-        # invalidate the cache for the redacted event
+    def _store_redaction(self, txn: LoggingTransaction, event: EventBase) -> None:
+        # Invalidate the caches for the redacted event, note that these caches
+        # are also cleared as part of event replication in _invalidate_caches_for_event.
         txn.call_after(self.store._invalidate_get_event_cache, event.redacts)
+        txn.call_after(self.store.get_relations_for_event.invalidate, (event.redacts,))
+        txn.call_after(self.store.get_applicable_edit.invalidate, (event.redacts,))
 
         self.db_pool.simple_upsert_txn(
             txn,
@@ -1812,9 +1815,7 @@ class PersistEventsStore:
             txn.call_after(self.store.get_applicable_edit.invalidate, (parent_id,))
 
         if rel_type == RelationTypes.THREAD:
-            txn.call_after(
-                self.store.get_thread_summary.invalidate, (parent_id, event.room_id)
-            )
+            txn.call_after(self.store.get_thread_summary.invalidate, (parent_id,))
             # It should be safe to only invalidate the cache if the user has not
             # previously participated in the thread, but that's difficult (and
             # potentially error-prone) so it is always invalidated.
diff --git a/synapse/storage/databases/main/relations.py b/synapse/storage/databases/main/relations.py
index 36aa1092f6..be1500092b 100644
--- a/synapse/storage/databases/main/relations.py
+++ b/synapse/storage/databases/main/relations.py
@@ -91,10 +91,11 @@ class RelationsWorkerStore(SQLBaseStore):
 
         self._msc3440_enabled = hs.config.experimental.msc3440_enabled
 
-    @cached(tree=True)
+    @cached(uncached_args=("event",), tree=True)
     async def get_relations_for_event(
         self,
         event_id: str,
+        event: EventBase,
         room_id: str,
         relation_type: Optional[str] = None,
         event_type: Optional[str] = None,
@@ -108,6 +109,7 @@ class RelationsWorkerStore(SQLBaseStore):
 
         Args:
             event_id: Fetch events that relate to this event ID.
+            event: The matching EventBase to event_id.
             room_id: The room the event belongs to.
             relation_type: Only fetch events with this relation type, if given.
             event_type: Only fetch events with this event type, if given.
@@ -122,9 +124,13 @@ class RelationsWorkerStore(SQLBaseStore):
             List of event IDs that match relations requested. The rows are of
             the form `{"event_id": "..."}`.
         """
+        # We don't use `event_id`, it's there so that we can cache based on
+        # it. The `event_id` must match the `event.event_id`.
+        assert event.event_id == event_id
 
         where_clause = ["relates_to_id = ?", "room_id = ?"]
-        where_args: List[Union[str, int]] = [event_id, room_id]
+        where_args: List[Union[str, int]] = [event.event_id, room_id]
+        is_redacted = event.internal_metadata.is_redacted()
 
         if relation_type is not None:
             where_clause.append("relation_type = ?")
@@ -157,7 +163,7 @@ class RelationsWorkerStore(SQLBaseStore):
             order = "ASC"
 
         sql = """
-            SELECT event_id, topological_ordering, stream_ordering
+            SELECT event_id, relation_type, topological_ordering, stream_ordering
             FROM event_relations
             INNER JOIN events USING (event_id)
             WHERE %s
@@ -178,9 +184,12 @@ class RelationsWorkerStore(SQLBaseStore):
             last_stream_id = None
             events = []
             for row in txn:
-                events.append({"event_id": row[0]})
-                last_topo_id = row[1]
-                last_stream_id = row[2]
+                # Do not include edits for redacted events as they leak event
+                # content.
+                if not is_redacted or row[1] != RelationTypes.REPLACE:
+                    events.append({"event_id": row[0]})
+                last_topo_id = row[2]
+                last_stream_id = row[3]
 
             # If there are more events, generate the next pagination key.
             next_token = None
@@ -776,7 +785,7 @@ class RelationsWorkerStore(SQLBaseStore):
             )
 
         references = await self.get_relations_for_event(
-            event_id, room_id, RelationTypes.REFERENCE, direction="f"
+            event_id, event, room_id, RelationTypes.REFERENCE, direction="f"
         )
         if references.chunk:
             aggregations.references = await references.to_dict(cast("DataStore", self))
@@ -797,41 +806,36 @@ class RelationsWorkerStore(SQLBaseStore):
             A map of event ID to the bundled aggregation for the event. Not all
             events may have bundled aggregations in the results.
         """
-        # The already processed event IDs. Tracked separately from the result
-        # since the result omits events which do not have bundled aggregations.
-        seen_event_ids = set()
-
-        # State events and redacted events do not get bundled aggregations.
-        events = [
-            event
-            for event in events
-            if not event.is_state() and not event.internal_metadata.is_redacted()
-        ]
+        # De-duplicate events by ID to handle the same event requested multiple times.
+        #
+        # State events do not get bundled aggregations.
+        events_by_id = {
+            event.event_id: event for event in events if not event.is_state()
+        }
 
         # event ID -> bundled aggregation in non-serialized form.
         results: Dict[str, BundledAggregations] = {}
 
         # Fetch other relations per event.
-        for event in events:
-            # De-duplicate events by ID to handle the same event requested multiple
-            # times. The caches that _get_bundled_aggregation_for_event use should
-            # capture this, but best to reduce work.
-            if event.event_id in seen_event_ids:
-                continue
-            seen_event_ids.add(event.event_id)
-
+        for event in events_by_id.values():
             event_result = await self._get_bundled_aggregation_for_event(event, user_id)
             if event_result:
                 results[event.event_id] = event_result
 
-        # Fetch any edits.
-        edits = await self._get_applicable_edits(seen_event_ids)
+        # Fetch any edits (but not for redacted events).
+        edits = await self._get_applicable_edits(
+            [
+                event_id
+                for event_id, event in events_by_id.items()
+                if not event.internal_metadata.is_redacted()
+            ]
+        )
         for event_id, edit in edits.items():
             results.setdefault(event_id, BundledAggregations()).replace = edit
 
         # Fetch thread summaries.
         if self._msc3440_enabled:
-            summaries = await self._get_thread_summaries(seen_event_ids)
+            summaries = await self._get_thread_summaries(events_by_id.keys())
             # Only fetch participated for a limited selection based on what had
             # summaries.
             participated = await self._get_threads_participated(